Osteopontin selectively regulates p70S6K/mTOR phosphorylation leading to NF-κB dependent AP-1-mediated ICAM-1 expression in breast cancer cells

In this study, we report that purified native OPN induces ICAM-1 expression in breast cancer cells. The data revealed that OPN induces NF-κB activation and NF-κB dependent ICAM-1 expression. We also observed that OPN-induced NF-κB further controls AP-1 transactivation, suggesting that there is cross talk between NF-κB and AP-1 which is unidirectional towards AP-1 that in turn regulates ICAM-1 expression in these cells. We also delineated the role of mTOR and p70S6 kinase in OPN-induced ICAM-1 expression. The study suggests that inhibition of mTOR by rapamycin augments whereas overexpression of mTOR/p70S6 kinase inhibits OPN-induced ICAM-1 expression. Moreover, overexpression of mTOR inhibits OPN-induced NF-κB and AP-1-DNA binding and transcriptional activity. However, rapamycin further enhanced these OPN-induced effects. We also report that OPN induces p70S6 kinase phosphorylation at Thr-421/Ser-424, but not at Thr-389 or Ser-371 and mTOR phosphorylation at Ser-2448. Overexpression of mTOR has no effect in regulation of OPN-induced phosphorylation of p70S6 kinase at Thr-421/Ser-424. Inhibition of mTOR by rapamycin attenuates Ser-371 phosphorylation but does not have any effect on Thr-389 and Thr-421/Ser-424 phosphorylation of p70S6 kinase. However, OPN-induced phosphorylation of p70S6 kinase at Thr-421/Ser-424 is being controlled by MEK/ERK pathway.These results suggest that blocking of OPN-induced ICAM-1 expression through mTOR/p70S6 kinase signaling pathway may be an important therapeutic strategy for the treatment of breast cancer.Breast cancer is one of the major causes of death among all other cancers in women globally. It emerges through a multi-step process starting from hyperplasia to premalignant change, in situ carcinoma, and invasive breast cancer [1-3]. Osteopontin (OPN), a calcified ECM associated non collagenous, sialic acid rich, glycosylated phosphoprotein is secreted by majority of the normal and transformed cells [4]. OPN isolated from different ce